Generally, as a hydraulic pressure source, hydraulic construction machines like hydraulic excavators are provided with a hydraulic pump, along with a rotational drive motor or a vehicle drive motor. For a hydraulic motor or hydraulic pump of this class, it has been known in the art to employ a swash plate type hydraulic rotational machine, for example, as disclosed in Japanese Patent Laid-Open No. H4-272482.
FIGS. 6 through 10 show examples of conventional swash plate type hydraulic rotational machine of the sort mentioned above, which are applied as a vehicle drive hydraulic motor.
More specifically, FIGS. 6 through 9 show a first example of the prior art. Indicated at 1 is a vehicle drive hydraulic motor which is constituted by a variable displacement swash plate type hydraulic rotational machine, for rotationally driving sprockets 35 through a speed reducer, which will be described hereinafter, which drive, for example, crawler belts (not shown) of a hydraulic excavator.
Designated at 2 is a casing of the hydraulic motor 1, the casing 2 being constituted by a main casing 3 and a rear casing 4 which closes an open axial end of the main casing 3 as shown in FIG. 7. The main casing 3 is formed in a cup-like shape which is open at one axial end and consists of a generally stepped cylindrical body portion 3A and a bottom portion 3B. Formed integrally with the outer periphery of the cylindrical body portion 3A is an annular flange 3C which is securely fixed to a truck frame (not shown) of the aforementioned hydraulic excavator by way of a number of screw holes 3D. Further, formed on the inner peripheral side of the stepped cylindrical body portion 3A is a double-step wall portion 3E which has its diameter increased in two steps toward the open end of the casing for mounting a brake device thereon.
Indicated at 5 is a drive shaft which is rotatably supported in the casing 2 as a rotational shaft, and at 6 is a rotor which is rotatably provided in the casing 2. This rotor 6 is splined on the outer periphery of the drive shaft 5 and located within the stepped body portion 3A of the main casing 3. Formed in the rotor 6 are a plural number of cylinders 7 which are extended in the axial direction at angularly spaced positions around the drive shaft 5. As will be described hereinafter, a piston 10 is reciprocably fitted in each one of the cylinders 7.
Denoted at 8 is a valve plate which is located between the rear casing 4 and the rotor 6 and securely fixed to the rear casing 4. This valve plate 8 is provided with a pair of supply and discharge ports 8A (only one of which is shown in the drawings) which are intermittently communicated with the respective cylinders 7 of the rotor 6. In turn, the supply and discharge ports 8A are communicated with a pair of supply and discharge passages 9 (only one of which is shown in the drawings) which are respectively formed in the rear casing 4.
Indicated at 10 are a plural number of pistons having one end portions thereof slidably fitted in the cylinders 7 of the rotor 6 and the other end portions projected out of the respective cylinders 7. The reference numeral 11 denotes a plural number of shoes which are swivelably mounted on the respective projected ends of the pistons 10. These shoes 11 are held in sliding contact with a swash plate 12, which will be described below, to guarantee smooth rotation of the rotor 6 relative to the swash plate 12.
Indicated at 12 is the swash plate which is tiltably mounted on the side of the bottom portion 3B of the main casing 3. The swash plate 12 is provided with an inclined surface 12A which is inclined relative to the center axis of the drive shaft 5 and held in sliding contact with the respective ones of the afore-mentioned shoes 11. The stroke lengths of the respective pistons 10, which determine the capacity of the hydraulic motor 1, are variable according to the inclination angle of the inclined surface 12A of the swash plate 12.
Indicated at 13 is a tilt support member which is provided on the side of the bottom portion 3B of the main casing 3 and formed in a semi-spherical shape for engagement with the back side of the swash plate 12. This tilt support member 13 serves as a fulcrum point in tilting the swash plate 12 to guarantees smooth tilting movements of the swash plate 12 on the side of the bottom portion 3B of the main casing 3. In this instance, the tilt support 13 is provided on each side of the drive shaft 5.
Indicated at 14 is a tilt actuator which is provided on the part of the bottom portion 3B of the main casing 3, and, as shown in FIG. 7, largely constituted by a pair of cylinders 15 which are formed axially on the side of the bottom portion 3B of the main casing 3 at spaced positions in the radial direction of the drive shaft 5, and a pair of tilt control pistons 16 which have respective base end portions slidably fitted in the cylinders 15, with the respective fore ends abutted against the back side of the swash plate 12.
By control pressures which are supplied to the respective cylinders 15 of the tilt actuator 14, through oil passage 17 which will be described hereinafter, one of the tilt control pistons 16 is extended out of its cylinder 15 while the other tilt control piston 16 is retracted into its cylinder 15. Thus, the tilt angle of the swash plate 12 is changeably controlled by the tilt actuator 14, by way of the respective tilt control pistons 16 which tilt the swash plate 12 about the tilt support portion 13 as a fulcrum point.
Indicated at 17 are oil passages which are bored through the main casing 3 to serve as passages for tilt control liquid pressures. These oil passage 17 are extended axially through the main casing body 3 in an askew or oblique fashion, and, at one end or at the open end of the cylindrical body portion 3A, are constantly communicated with oil passages 18 and 19 which will be described hereinafter. At the other end, the passages 17 are communicated with the cylinders 15 of the tilt actuator 14 to supply control pressures to and from the cylinders 15.
Denoted at 18 and 19 are oil passages which are formed in the rear casing 4, and denoted at 20 is a volumetric control valve which is provided in the rear casing between the oil passages 18 and 19. In this instance, the volumetric control valve 20 is manually switched by an operator of the hydraulic excavator or the like to supply part of oil pressure in the afore-mentioned oil passage 9 selectively to the oil passages 18 and 19. Thus, the tilt angle of the swash plate 12 is variably controlled by the tilt actuator 14 as mentioned hereinbefore, by supplying a control pressure of high level to one of the oil passage 17 which are in communication with these oil passage 18 and 19, while holding the other oil passage 17 at a lower pressure level.
Indicated at 21 is a negative type brake device which is located between the main casing 3 and the rotor 6 for applying brakes on the rotor 6 and rotational shaft 5. In this instance, as shown in FIG. 7, the negative type brake device 21 is constituted by; an annular stopper 22 which is fixedly mounted on the stepped wall portion 3E on the inner periphery of the stepped cylindrical body portion 3A of the main casing 3; a plural number of brake plates 23 which are mounted on the stepped wall portion 3E adjacent to the stopper 22 for movements in the axial direction but blocked against movements in the rotational direction; a plural number of friction plates 24 which are interposed between the respective brake plates 23 and mounted on the outer periphery of the rotor 6 for movements in the axial direction but are blocked against movements in the rotational direction; a brake piston 25 which is slidably fitted in the stepped wall portion 3E on the side of the open end of the cylindrical body portion 3A; a spring 26 which is interposed between the rear casing 4 and the brake piston 25 to bias the brake piston 25 constantly toward the brake plates 23; and a liquid pressure chamber 27.
Under the influence of the biasing force of the spring 26, the brake device 21 functions to hold the brake plates 23 in frictional contact with the respective friction plates 24 between the brake piston 25 and the stopper 22, applying the so-called parking brakes arresting the rotor 6 along with the drive shaft 5.
Designated at 27 is a hydraulic chamber which constitutes part of the brake device 21. As shown in FIG. 8, the hydraulic chamber 27 is formed between the cylindrical body portion 3A of the main casing 3 and the brake piston 25, and supplied with a brake cancellation pressure through oil passages 28 and 29 which will be described hereinafter. As the brake cancellation pressure in the hydraulic chamber 27 rises to overcome a preset biasing force of the spring 26, the piston 25 is thereby pushed against the action of the spring 26. As a result, the respective brake plates 23 are pushed into positions which are slightly spaced from the friction plates 24 to release the rotor 6 and drive shaft 5.
Designated at 28 and 29 are oil passages which convey brake cancellation pressure to be fed to and from the hydraulic chamber 27 of the brake device 21. Of these oil passages 28 and 29, as shown FIG. 8 the oil passage 28 is extended obliquely through the cylindrical body portion 3A of the main casing 3, and communicated with the oil passage 29 at one end thereof on the side of the open end of the cylindrical body portion 3A and communicated with the hydraulic chamber 27 at the other end. On the other hand, the oil passage 29 is formed in the rear casing 4, and connected to one of the aforementioned paired supply and discharge passages 9 which is at a high pressure level, through a high pressure selector valve such as a shuttle valve (not shown).
In this instance, at the time of rotationally driving the hydraulic motor 1, the hydraulic fluid which is supplied to the hydraulic motor 1 from an oil pressure source (not shown) through a directional change-over valve is led to the oil passage 29 through the afore-mentioned high pressure selector valve. Further, this hydraulic fluid is supplied as a brake cancellation pressure to the hydraulic chamber 27 through the passages 28, so that the brake device 21 releases the brakes on the rotor 6 and the drive shaft 5, permitting to start the hydraulic motor 1.
Further, at the time of stopping rotation of the hydraulic motor 1, the supply of hydraulic fluid from the oil pressure source is blocked by the above-mentioned directional change-over valve, whereupon the oil pressure (the brake cancellation pressure) in supply to the passages 29 and 28 through the high pressure selector valve drops down to the level of tank pressure. Therefore, the brake device 21 is applied by the action of the spring 26 as described above, braking the rotor 6 and the drive shaft 5 against rotation.
Indicated at 31 is a reducer for the vehicle drive, which is provided in the main casing 3 of the hydraulic motor 1 as shown in FIG. 6. This reducer 31 is largely constituted by a housing 32 of cylindrical cup-like shape which is rotatably mounted on the side of the bottom portion 3B of the main casing 3, and two-stage planetary gear systems 33 and 34 which are provided in the housing 32. A sprocket 35 is mounted on the outer peripheral side of the housing 32 to serve as a drive wheel.
Further, provided within and on the center axis of the housing 32 of the reducer 31 is a rotational shaft 36 which is splined with the drive shaft 5 of the hydraulic motor 1 for rotation therewith. As the rotational shaft 36 is driven by rotation of the hydraulic motor 1, its rotation is transmitted to and reduced through the planetary gear system 33 of the first stage, and then further reduced through the planetary gear system 34 of the second stage. In this instance, by rotation of the housing 32, rotation of large torque is transmitted to the sprocket 35.
With the hydraulic motor 1, which is constituted by a conventional swash plate type hydraulic rotational machine of the construction as described above, the hydraulic fluid in supply and discharge to the hydraulic motor 1 from an oil pressure source is fed to and from the respective cylinders 7 of the rotor 6 through the supply and discharge passages 9 in the rear casing 4 and through the supply and discharge ports 8A in the valve plate 8. As a result, pushing force is generated in each one of the pistons 10, acting against the swash plate 12 through the shoes 11. By this pushing force, the respective shoes 11 are glided on and along the inclined surface 12A of the swash plate 12 to rotate the rotor 6 integrally therewith through the pistons 10. This rotation of the rotor is transmitted to the reducer 31 through the drive shaft 5.
At this time, if the volumetric control valve 20 is switched by the operator of the hydraulic excavator, part of the hydraulic fluid in supply to the afore-mentioned supply and discharge passages 9 is selectively supplied to either one of the passages 18 and 19 as a control pressure. By this operation, a control pressure of high level is supplied to one of the passage 17, which are in communication with the passages 18 and 19, while the other one of the passage 17 remains at a low pressure level. This control pressure causes one of the tilt control pistons 16 of the tilt actuator 14 to extend out of its cylinder 15 while retracting the other piston 16 into its cylinder 15.
As a result, by the tilt control pistons 16, the swash plate 12 is turned about the tilt support portion 13 to move into a tilted position of a different angle. Namely, the tilt angle of the swash plate 12 is variably controllable by way of the tilt actuator 14. When the swash plate 12 is set at a maximum tilt angle, each piston 10 is displaced over a maximum stroke distance. In this position, the flow rate which is necessary for rotating the rotor 6 is increased, permitting to rotate the drive shaft at low speed and with high torque. On the other hand, when the swash plate 12 is set at a minimum tilt angle, each piston 10 is displaced over a minimum stroke length. In this position, the flow rate which is necessary for rotating the rotor 6 is reduced, permitting to rotate the drive shaft 5 at high speed and with low torque.
Illustrated in FIG. 9 is a casting stage in a process for fabricating a cast structural material for the main casing 3 of the hydraulic motor 1.
A cast structural material 46 is produced by the use of a casting mold set 41, i.e., a split mold set consisting of upper and lower mold sections 42 and 43, which are butt-joined one on the other, and a core 44. This mold is, for example, a sand mold which is formed of casting sand or the like. The upper and lower mold sections 42 and 43 are internally provided with cavities 42A and 43A, respectively. Formed in the upper mold 42 is a sprue 45, through which molten metallic material F is poured into the mold 41. Further, the core 44, which is set in position between the upper and lower molds 42 and 43 is provided with cylindrical projections 44A at its upper end, at positions which correspond to the respective cylinders 15 of the tilt actuator 14.
In the state as shown in FIG. 9, molten metallic material F is introduced into the casting mold 41 in the arrowed direction, and then allowed to solidify to shape with gradual cooling to obtain a cast structural material 46 of a shape which is defined by the mold cavities 42A and 43 and the core 44.
Nextly, after ejection of the cast structural material 46 from the mold 41, a main casing 3 for the hydraulic motor 1 is formed out of the cast structural material 46 by removing outer peripheral surfaces of the structural material 46 by machining up to the positions as indicated by two-dot chain lines in FIG. 9. Then, drilled holes 47 of narrow elongated form are bored axially in an obliquely inclined fashion through the cast structural material 46, from one axial end toward the other axial end thereof, to provide the passage 17 or pressurized liquid passages.
On the other hand, according to a second prior art shown in FIG. 10, a cast structural material 56 is produced by the use of a casting mold 51, e.g., a sand mold consisting of upper and lower molds 52 and 53 and a core 54. The upper mold 52 is provided with a sprue 55 for pouring a molten metallic material F into the mold 51. Further, the core 55 which is set in position between the upper and lower molds 52 and 53 is provided with cylindrical projections 54A at its upper end, more specifically, at positions which correspond to the aforementioned cylinders 15.
Molten metallic material F is poured into the casting mold 51 of FIG. 10 in the arrowed direction, and, after obtaining a cast structural material 56, a main casing 3' is form out of the cast structural material similarly by machining same, i.e., by machining outer peripheral surfaces of the casting 56 up to the positions indicated by two-dot chain lines in FIG. 10. Then, a plural number of drilled holes 57, including narrow and elongated drilled holes 57A to 57D, are bored through the cast structural material 56 to provide the passage 17'.
According to the firstly mentioned prior art, after casting the structural material 46 for the main casing 3 by the use of the casting mold 41, inner and outer peripheral surfaces of the cast structural material 46 are machined down to the shape of the main casing 3 of hydraulic motor 1. Then, elongated narrow drilled holes 47 are bored through the cast structural material 46 obliquely in the axial direction from one to the other end thereof to provide the passage 17 as described above.
Therefore, according to the first prior art, the cast structural material 46 (the main casing 3) needs to have an increased wall thickness in those regions which contain the drilled holes 47, in order to prevent the drilled holes from breaking out through the wall of the cast structural material 46 in the course of drilling operations. However, in case the wall thickness of the cast structural material 46 is increased, casting defects are likely to occur due to large variations in wall thickness around the projections 44A which are formed on the core 44.
Such casting defects, if remain in the main casing 3, can invite a problem of oil leakage from the oil conduits 17 which convey high hydraulic fluid. Further, due to the necessity for boring the drilled holes 47 in the cast structural material 46 for the passage 17, the drilling operation consumes a great deal of time and labor to such a degree as to deteriorate the operational efficiency to a considerable degree in manufacturing main casings 3 from cast structural material 46.
Further, in the case of the cast structural material 46 by the prior art shown in FIG. 9, the wall thickness as well as the axial length of the cast structural material 46 has to be increased in order to bore the drilled holes 47 linearly in oblique directions. Therefore, when a reducer 31 is mounted on a main casing 3 as shown in FIG. 6, the rotational machine as a whole has a large length which may give rise to various problems, for example, damages to the housing 32 of the reducer 31 as caused by jumping stones or rocks when end portions of the reducer 31 are protruded on the outer side of a crawler belt.
On the other hand, according to the second prior art shown in FIG. 10, it is possible to reduce the axial length of the cast structural material 56. In this case, however, a plural number of drilled holes 57 have to be bored through the cast structural material, more specifically, a plural number of narrow and elongated drilled holes 57A to 57D for use as the passage 17'.
The boring operations for the drilled holes 57 of this nature require higher precision work and extra time and labor in order to bring the elongated narrow drilled holes 57A to 57D into predetermined aligned positions at the respective fore ends. Besides, of the elongated narrow holes 57A to 57D, for example, the ends of the narrow elongated holes 57B to 57D which are open to the outside of the cast structural material 56, have to be closed with plugs despite high probabiilities of oil leakage through plugged ends.